1. Technical Field of the Invention
The present invention relates to an electrosurgical apparatus of the type which is used, in particular by surgeons and dentists, to perform incisions and also to stop bleeding.
2. Prior Art
Electrosurgical equipment of the type in question is based on a power generator which supplies a high frequency alternating current. Via a connection cable this current is conducted to an electrode—the so-called active electrode—which is replacably mounted in a handpiece. For dissections the active electrode is usually made of thin metal wire which remains cold because of good conductivity, but when the electrode is advanced through tissue, a local heating is generated in the tissue a. o. because of the electrical resistance of the tissue.
This heating causes a molecular dissolution of the tissue cells since the generated heat causes both intra- and extracellular generation of steam which bursts the tissue cells, and the result is a well-defined incision in the tissue. How deep this dissolution takes place in the tissue is depending a. o. on the current intensity and on the speed at which the electrode is advanced through the tissue. Normally the faces of incision are substantially free of bleeding, and the damages to adjacent tissues are at a minimum.
Because of the restraining effects to bleeding, the equipment can also be used specifically to stop bleeding. For that purpose the generator is usually switched to provide a modulated current waveform, and the result is a deeper coagulation necrosis relative to the electrode which usually is ball-shaped for that purpose. The result is an efficient hemostasis in connection with concentrated as well as more diffuse bleeding.
An important benefit of using electrosurgery instead of a scalpel is a substantially more precise incision and, moreover, the operating field remains clean, dry and without bleeding. This is an essential advantage e.g. in connection with the taking of impressions for tooth crowns and bridges, because an impression can be taken immediately after after that the necessary incisions have been made, Electrosurgery is also well-suited for pre-prosthetic surgery and for paradontal surgery as well.
In electrosurgery one of the principal goals is to make clean cuts with a minimum of tissue destruction. This requires that the output power from a cutting electrode continously is precisely adapted to the type of tissue and to the amount of issue (the depth of cut) with which the electrode is in contact. At the same time the output power may at no instant be larger than necessary, because too large power delivery will result a.o. in unclean cuts and possibly also in sparkling between electrode and tissue which causes tissue necrosis and thereby complicates and prolonges the healing proces.
In particular the output power must be effectively and rapidly reduced as soon as a cutting electrode approaches the periosteum or bone tissue. Otherwise, the result would be very painful damages such as periostitis, bone necrosis and sequestration of bone tissue.
In other words, electrosugical equipment of the type under consideration for making incisions in living tissues must include very efficient means for continuous control or dosage of the power supplied by a cutting electrode, Moreover, such power control means must respond rapidly—virtually instantly—to changes in the electrical load of the electrode i.e. the conductivity or susceptance.
The electrical load is to an essential extent determined by two factors, namely the material resistance i.e. the inherent resistance of the tissue, and the conducting area i.e. the area of contact between electrode and tissue (the depth of cut).
Both factors may vary during a cutting operation. If for instance an electrode which is advanced at constant depth through soft tissue, approaches bone tissue, then there will be a sudden decrease in the load or susceptance experienced by the electrode, because bone tissue has a substantially higher electrical resistance than soft tissue due to the substantially lower contents of liquid in bone tissue. As already mentioned a very rapid reduction of the power supplied by the electrode must take place in such instances, because, otherwise, the result could be very painful damages to bone tissue or tooth substances.
If on the other hand the cutting depth of the electrode is suddenly decreased, then there will also be a sudden decrease in the load or susceptance due to the decreasing area of contact between tissue and electrode. The minimum load or susceptance which is zero, at least in principles, will occur when the electrode gets free the tissue—and thus is in the air. Again it is very important that the power from the electrode is decreased very rapidly, because otherwise the result could be sparkling and serious burning and destruction of tissue.
In conclusion, it is important that the power supplied by a cutting electrode is efficiently and rapidly decreased as soon as the load or susceptance experienced by the electrode is decreased for one reason or another.
DK patent No. 149 762 discloses an electrosurgical apparatus of the type under consideration and which provides an extremely rapidly and precisely responding control of the power output from the electrode as a function of the instant load on the electrode. This control has been achieved by using impedance matching techniques, and the system is matched or in tune at one quite specific load impedance which occurs when an electrode is cutting at maximum depth in soft tissue.
In or immediately around this particular matched condition, the output power from the electrode will accordingly be at its maximum, whereas the output power will be efficiently and rapidly decreased because of mismatching in all other conditions i.e. with decreasing depth of cut and/or with increasing electrical tissue resistance.
Thus, impedance matching technique has been utilized to deliberately mis-match the output stage in such a manner that the oscillation ability, and thereby the power output, is reduced as soon as the electrode load moves away from the particular condition with impedance matching. The result is an automatic and direct regulation in true time of the power output as a function of the instantaneous load and in such a manner that the power delivered to the tissue by a cutting electrode, continuously is adapted to the type of tissue and/or to the depth of cut.
However, in the electosurgical apparatus just described it has been seen that the regulation range for output power is not sufficiently broad to range—without further measures—all the way from cutting in the tissue surface without sparkling (minimum load) and up to cutting in full depth with maximum power (maximum load). It has therefor often been necessary to compromise either by renouncing on a part of the available maximum power in return of a completely sparkless operation, or by accepting some degree of sparkling in return of availability of the full maximum power.